1. Field of the Invention
The present invention relates to an improved method of making semiconductor device comprising a layer of highly accurate thickness with a sufficient reproducibility and an easily controllable process.
2. Description of the Prior Art
Hitherto, in making a compound semiconductor device employing a GaAS or the like, it has been known that a thickness control is a very important problem for an epitaxially grown layer formed on a semiinsulative or of high resistivity semiconductor layer or insulative substrate. For example, in Schottky gate field-effect transistors (hereinafter called MESFETs) employing GaAs, which recently become widely to be used, it is necessary that distribution of thickness of the semiconductor substrate must be suppressed within .+-.0.02 .mu.m for the thickness of 0.25 .mu.m. Such accurate control of the thickness has been almost impossible even though by utilizing known epitaxial growth technique as well as chemical etching technique. In order to ease such difficulty, a new technique called anodic oxidation (or anodization) in an electrolyte is recently being developed. This is the process wherein etching of the active region is made by forming anodized film of a relatively highly accurate thickness in the active region by controlling current or voltage, followed by removing the oxide film thereby to suppress distribution of thickness of the resultant active region. However, the anodic oxidation method also has a problem that the active region as initially formed in the first step must have uniformly accurate thickness on the whole area of the substrate and such requirement is substantially impossible to perform.
Thus, hitherto, forming of a highly accurate thickness active region has been a very difficult problem.